Adhesion molecules on the surface of monocytes have been shown to be important in the triggering of IL-1 and TNF and have also been shown to have a role in the killing of tumor cells by monocytes. Because of their role in these and most probably other monocyte functions, we studied the regulation of expression of adhesion molecules on monocytes by cytokines. Purified human monocytes were found to respond to various cytokine treatments with changes in the expression of the adhesion proteins: MAC-1, pl5O,95, ICAM-1 and CD44. Basal expression of ICAM-1 was enhanced by IFN-gamma, TNF, IL-4 and GM-CSF while CD44 expression was increased by TNF, IL-1 and GM-CSF. pl5O,95 was affected only by GM-CSF treatment. Both GM-CSF and IL-4 enhanced MAC-1 expression while IFN-gamma and TNF lowered MAC-1 expression. None of the cytokines altered LFA-1 or LFA-3 expression. Inducing changes in the expression of these proteins may be one mechanism by which cytokines regulate the functions of monocytes in particular and immune responses in general.